Signal Transducers and Activators of Transcription (STATs) are intracellular signaling proteins that are activated upon tyrosine phosphorylation and translocate from the cytosol to the nucleus where they bind to specific regions of DNA and exert transcriptional activity. We have accumulated convincing evidence that constitutive activation of STATs 3 and 5 in squamous cell carcinoma of the head and neck (SCCHN) represent critical survival pathways in SCCHN, whereas Stat1 activation inhibits tumor growth. We recently reported that targeting Stat5b, but not Stat5a, abrogated SCCHN growth in vitro. Results of preliminary studies demonstrate that Stat5b activation is linked to SCCHN tumorigenesis in vivo. Potential causes of constitutive STAT activation include autocrine stimulation of upstream receptor and non-receptor kinases. Previously, we have shown that STATs are activated following stimulation of EGFR in SCCHN. Further investigation implicates Src family kinases in EGFR/STAT activation mechanisms in these cells. Therefore, we propose to test the hypothesis that loss of growth control in SCCHN is mediated through acquisition of a TGF-alpha/EGFR autocrine signaling pathway that is mediated by selective activation of STATs, with the ultimate aim of designing novel treatment strategies to target these pathways. To accomplish the goals of this renewal application, we will: 1) examine the differential function of Stat5 isoforms in normal and transformed mucosal squamous epithelial cells; 2) determine the role of Src kinases in EGFR-mediated STAT signaling pathways in SCCHN; and 3) determine the mechanisms by which Stat1 functions as a tumor suppressor in SCCHN.